Field of the Invention
The invention relates to a device for discharging liquid from a water separator in a fuel cell system according to the kind defined in greater detail in the preamble of claim 1. The invention further relates to a method for discharging liquid from a water separator in a fuel cell system by means of such a device.
Description of the Related Art
Fuel cell systems are known from the prior art. The functionality is in essence that electrical energy and product water is produced from hydrogen and oxygen. Here, the resulting product water together with exhaust gases is discharged from the region of the fuel cell. In order to separate the liquid water from these exhaust gases, water separators are typically provided in the fuel cell systems. Hereby, on the one side, liquid water can be prevented from escaping from the fuel cell system and, on the other, potential freezing of line elements due to said liquid water can be prevented if the fuel cell system is operated at temperatures below the freezing point and, in particular, if it is turned off. If exhaust gases of the fuel cell system are at least partially recirculated to the fuel cell, thus, the fuel cell system is provided with a so-called cathode and/or anode recirculation, water separators are typically also provided in the recirculation lines so that the resulting product water can be separated and is not recirculated again into the fuel cell. Here, liquid water could wet parts of the fuel cell, which would adversely affect the functionality of the fuel cell.
In particular in the case of water separators in the region of the anode of the fuel cell system it is now of vital importance that the water is discharged from time to time and that, in addition, the (residual) hydrogen present in products and reactants on the anode side cannot escape, or only in minimal amounts, into the surrounding area.
Devices for discharging liquid water are known from US 2006/0088756 A1, which devices comprise a capacitive sensor, a controller and a valve device activated by said controller. The capacitive sensor is located upstream of the valve device in the flow direction of the water to be discharged. When the valve device is closed as soon as there is no more liquid in the region of the capacitive sensor, it is then possible, with a suitable spacing between the sensor and the valve device downstream of the sensor, that leakage of gas can be completely prevented or, with an appropriately smaller spacing, can be reduced to a tolerable minimum.
The problem of this construction is that capacitive sensors for detecting liquids in fuel cell systems unfortunately have not proven their worth. It became apparent over the years in development and research systems that capacitive sensors, for example as level sensors in a water separator, or as used in the mentioned US publication, are extremely susceptible to failures. This phenomenon occurs in particular in fuel cell systems since due to the substances and contents of the liquid water involved here, the capacitive sensors are extremely difficult to use and become soiled rather quickly.